The reasons are: - the blood is very low on nutrition - the leech is not feeding itself regularly
The leech slows it metabolism a lot, so it can survive on very low nutrition. The blood is very low on nutrition, and it is the leech's food source, so the evolution made it preserve energy.
The leach doesn't have the opportunity to feed itself on regular basis, so it takes maximum advantage of any opportunity and consumes as much blood as possible so that it can survive long term when it will not have food at disposal.
If a female cat with orange fur (XOXO) is crossed with a male cat with black fur (XoY) then:
P: XOXO x XoY
F1: XOXo XOY XOXo XOY
This means that half of the offspring will be females with tortoiseshell color (XOXo) and other half of the offspring will be males with orange fur (XOY).
